An update to my earlier post (2015 iMac 27" with fusion drive): The following steps worked for me:
Boot with option key down into a Big Sur installer USB flash drive
From terminal, execute 'diskutil resetfusion' to wipe, reformat, and rebuild, the built in fusion drive
Return to main recovery menu and install Big Sur from the USB flash drive.
No noticeable slow down.
jonathon-b
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution
Last seen
User for
From
Cambridge, MA, United States
Post
Replies
Boosts
Views
Activity
For iMac users seeing a slow down with Big Sur that are *not* running Avast: Does your iMac have a fusion drive? (large spinning disk, small SSD cache 'in front' of it)
Did you upgrade from Catalina, from an older OS like Mojave, or did you perform a 'clean-install' of Big Sur by booting from a flash drive installer and reformatting the hard drive?
Is it possible that the fusion drive configuration has been messed up by the upgrade / reformat process?
If so, then your iMac could be effectively running directly from the hard drive alone - without the benefit of the SSD cache - and the fix would be to rebuild the fusion drive and reinstall from scratch.